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OMG guru Dave K continues the Robot Arms project. But first, our hero must remove offensive odors...with SCIENCE! -Travis
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I've found that the fat cables that came with the robot arm I got from the Makery are out-gassing a horrid stink of damp basement. Whelwick is starting to smell like an old barn. To address this I'm going to take a two phase approach. I'll need to address the source of the odor, the cables, and also, mostly for fun, I'll build an ozone generator to eradicate the airborn odor through oxidation of whatever fumes it's been putting in the air here.

To treat the cables I intend to soak them in bleach solution for a few days. Before I do this, I want to leave the ozone generator running for a while, to see how effective it is at neutralizing the odor causing particles.

The simplest way to generate ozone, at least for me, is to use the corona discharge method to convert atmospheric oxygen, O2 to ozone, O3. I'll be using a 12kV 30mA neon sign transformer. This operates at the line frequency of 60Hz, which isn't great for ozone production, but it does work. Another way to do this is to repurpose the flyback transformer from an old CRT (I have half a dozen if anyone needs such a device). The flyback can be driven from a simple 555 or microcontroller circuit to produce a high voltage, high frequency signal which is more effective than 60Hz for ozone production. It's also smaller.

To produce the corona discharge I'm going to try using conductive plates separated by an insulator. In this case, aluminum ductwork tape and glass. First I cut a bit of glass from a piece of scrap.

I then applied the tape to both sides and trimmed it clear of the edges to avoid arcing around the edges.

The corona discharge occurs at the edges of the plate, not where they overlap, so I want as much edge as I can get. Overlap provides capacitance, which for this application, I don't need or want, so I want a plate pattern with no overlap and plenty of edge. I went with an 'E' pattern. It's easy to cut.

I decided to use copper wire from a length of Romex 10/3 as connectors, so I could easily support the panel with the connectors. To maximize the contact area I made the wire long and gave it a right angle bend to prevent it twisting.

I secured the connectors with some Gorilla Tape (it was what was handy, and it's fun to use).

The corona panel is then connected to the transformer. For testing I've attached the neon sign transformer to a variable autotransformer so that I can start out at a low voltage and ramp up the output of the neon sign transformer. Too much voltage can puncture the glass dielectric, which will shatter the glass (another reason for the tape, to contain the glass). I don't think there is much danger of this, since I'm not reaching voltages where this should be a problem, but if there are flaws in the glass it is possible.

The wiring for the neon sign transformer also has a safety gap configured. The spacing of the gap is set so that any voltage much above the 12kV the transformer secondary is designed for will cause an arc across the safety gap to protect the transformer.

To verify that it is generating ozone, and just in case it bursts, I put a container over the corona panel until I ran it up to full voltage and let it sit for a while.

The corona is fairly faint unless your eyes are dark adapted. To get visualize it I've set the camera up for long exposure in the dark. My cameras are cheapie point-n-clicks, so you'll have to put up with the not-so-great quality. here it is from both sides.

Here's what happens if you up the voltage but forget to open up the safety gap.

From the corona images it is apparent that the sharp points of the 'E' pattern are producing a lot of corona (the shape of an electric field around sharp points encourage dielectric breakdown and corona discharge, this is why bulbous, polished surfaces are so popular in high voltage labs). If you look closely though, you can see that there is discharge along the edges as well, as was intended. I don't know which is better. Regardless, it generates enough ozone. The staticy odor of ozone is strong after running for a couple of seconds of running.

We put together a little Ruben's tube at the OMG meeting today. There has been some discussion about building a large tube for a week or so, but no one has yet come up with a length of tubing to use. We were standing around talking after the meeting and someone observed that the remnants of a small telescope that was sitting on the table could probably be used to make a small Ruben's tube. So we scrounged up some additional parts from the parts bins (pc speaker, vinyl hose, bread bag, aluminum sheet, duct tape) and whipped up this little guy.

One of the guys got some good video of it running, which I expect will show up on the Omaha Maker Group website or mailing list before too long.

Tonight I built a really crappy coupler to connect the motor on the cat feeder mechanism to the drive screw on the slide. I was having some difficulty getting anything to work and started on a new slide with a scotch yoke design, but, not surprisingly, I had trouble connecting to the motor shaft for that too. So I poked around a bit and found a grub screw in the junk box that I figured would work to secure an adapter to the motor shaft. To make the adapter I cut a short length of aluminum rod which I then needed to bore out to the diameters of the two shafts.

Since I don't have the equipment to properly bore a shaft, I did it improperly, and with really poor quality. To do this I mounted the rod in the drill press chuck, put a drill bit into the vise, and made an attempt to set the bit vertical and on center, then drilled away. This technique can be used to put a hole exactly in the center of the rotating shaft, give or take a few inches. Flip and repeat with the smaller bit for the motor shaft.

I tapped the larger hole for the screw, then mounted it up and marked where the motor shaft flat was so I could drill a hole for the grub screw. I didn't have a tap small enough for the grub screw, but since the screw is steel and the shaft is aluminum, it can self-tap. I ground the outside of the shaft a bit to remove some of the worst of the imbalance from the crappy, off-center boring job, dripped some loctight in it, and mounted it all up. I'm too impatient to wait for loctight to dry though, so I crushed the threaded portion to secure it to the shaft.

It's working pretty well. I'm not sure how much torque this motor generates, but at stall it generates a reasonable amount of linear force through the screw, a pound or two. At stall the motor draws 2.5A, so I'll need to scrounge up some appropriate transistors for the h-bridge.

I spent the evening working on the mechanicals for an automatic dry cat food dispenser. It consists of a screw-driven slide with a 2 inch hole. The assembly will be placed under a large cat food bucket with a hole in the bottom, with a chute directing dispensed cat food into a bowl.

At the moment the screw is driving the slide simply by threads cut directly into the wood. If necessary I will embed a nut in the slide, but I'm hoping the forces will be low enough that the wood will be sufficient. I've got the motor coupled directly to the screw. This means I'm using a far larger motor than is really necessary for this job. Unfortuantely I didn't have any gear drives that were really suitable. Also unfortunately I didn't have any reasonable way to couple the motor to the screw, so I just bored the end of the screw out to the diameter of the motor shaft and put some Loctight in it. This obviously won't last long, but it was a quick fix. I'll need to come up with something interesting to couple the shafts. Maybe it would be interesting to cast a three-part coupler for this.

The next part of the project will be the electronics for driving the motor. This will consist mostly of some end switches for the slide and an h-bridge driver for the motor. I'll probably put an AVR into it, just because they're cheap and I've got a bunch lying around looking for work. On the other hand, since it just needs to cycle once per input trigger, it might be fun to do it with something more rudimentary with some basic logic.

I picked up some casting sand from a local aluminum foundry this weekend and spent Sunday putting together a small test casting. Rather than using aluminum, which is a challenge to melt in a soup can, I used a few rolls of pennies as a source for some zinc.

It took a few tries to get the cope rammed up properly. I had trouble getting the sand inside the pattern rammed hard enough and strongly connected to the rest of the sand in the cope. It did work out nicely though. I took some pictures of the casting process and the resulting object.

Spent the day putting together a bench for the Omaha Maker Group Spectralight milling machine. I took way too long tearing apart a pallet some of the guys pulled out of a dumpster and reassembling it as a table. Then we mounted the mill and started troubleshooting the crappy stepping.

We had the mill connected up to grbl and that worked pretty well, but when we connected it to the PC via the hacked up parallel to Spectralight cable, it gave really jerky movement. After an hour of poking around we figured out that the PC parallel port just doesn't have enough drive for the controller. That's a pretty common problem with on-board parallel ports, but I was hoping it would be able to drive the optoisolators in the Spectralight controller.

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